In order to reduce the detent torque and the torque ripple of an electric drive of, for example, a brushless synchronous motor excited by a permanent magnet, in which the rotor has permanent magnets, the rotor pole shoes of the rotor are provided with a specific shape. The shaping of the rotor pole shoes occurs in such a way that an air gap flux density distribution results, which is as sinusoidal as possible.
Up to now the outer edge of the pole shoe has for that reason been provided with a contour, by which an air gap is formed between two adjacent rotor pole shoes and which starting from the center of the rotor shoe pole extends substantially according to an inverse cosine function. A contour of a rotor pole shoe, which is formed in this manner, is referred to as a so-called Richter contour (Richter, R., “Elektrische Maschinen” (‘Electric Machines’), Vol. I, “Allgemeine Berechnungselemente, Die Gleischstrommaschine” (‘Common Computing Elements, The Direct Current Machine’), 3. edition, Birkhäuser Publishing House, Basel 1967, pages 168-170).
The Richter contour produces an air gap flux density distribution having a distortion factor of typically less than 2% and therefore an ideal sinusoidal air gap flux density distribution. The manufacture of a rotor with a Richter contour is complex in design and manufacture, and therefore an arc contour is used for the outer contour of the rotor pole shoes to simplify the process. The arc contour is defined by an outer radius of a circle-segment, which has a smaller radius than the rotor outside diameter and whose center is offset with respect to the shaft of the rotor along the centerline of the relevant rotor pole shoe. Such a so-called off center arc contour is a simple approximation of the Richter contour and approximately produces an air gap flux density distribution having a distortion factor of approximately 5%. This does in fact lead to a somewhat higher detent torque and a higher torque ripple than is the case for a rotor having a Richter contour. The design and the manufacture of such a rotor arrangement are however substantially simplified.
The arc radius, which is optimal for an air gap flux density distribution that is as sinusoidal as possible, is however not known when using the off center arc contour and its offset from the shaft center.
It is therefore the aim of the present invention to provide an electric machine as well as a rotor arrangement for an electric machine, whose rotor pole shoes have an off center arc contour, the rotor arrangement being optimized with regard to an air gap flux density distribution, which is as sinusoidal as possible.